A synthesizer creates a stable local oscillator (LO) to enable frequency conversion in radar/communication systems. By changing the frequency of the local oscillator, the system can tune across various frequencies. A key technical performance measure for frequency synthesizers is single-sideband phase-noise. Radar systems' detection and identification capability is directly proportional to the phase-noise performance of the LOs that are used for frequency conversion. The mechanism for tuning this LO signal resides within the synthesizer itself, an example of which is a direct digital synthesizer (DDS). The tuning mechanism within a frequency synthesizer degrades phase-noise performance based on the degree of tuning resolution. In other words, a higher degree of tuning (i.e., smaller step size) causes more degradation. When architecting a radar system, detection/identification capability often has to be sacrificed for frequency agility.
Synthesizers can be classified into 3 categories: phase-locked loop (PLL)-based, DDS-based, and mix-and-multiply. In terms of phase-noise performance, the latter two architectures provide the best phase-noise. DDS-based synthesizers offer high resolution with good residual phase-noise. A typical DDS is clocked with a high-frequency reference input, coupled with a numerically controlled oscillator (NCO) to generate a controlled frequency and phase output signal. The DDS output range is dictated by this NCO word. Generally, the output frequency can be tuned up to about 40% of the reference frequency. As an example, if a DDS can be clocked with up to a 2 GHz input reference, it is capable of an output up to about 800 MHz. In general, a (DDS) can provide very agile tunability, however the total phase-noise is degraded in a DDS. Therefore, a DDS-based synthesizer architecture will generate higher phase-noise when compared to a mix-and-multiply synthesizer architecture.
A mix-and-multiply synthesizer architecture may provide the best possible phase-noise performance, but it is prohibitively expensive and large to implement. A mix-and-multiply synthesizer generates discrete offset frequencies without using a DDS device, which therefore circumvents the additive phase-noise of the DDS completely. These synthesizers require multiple RF modules to generate, filter, and condition the frequency offsets that are switched across the desired radar band.